6BTM or locked out timing?

I'm not very familiar with the guts of the S6 distributors but I have to imagine that they are similar to their v8 cousins in that they have springs and weights that control the timing curve advance and those are the components that must be "locked" in order to fix your timing at a particular setting. That would be the cheapest route. Locked distributor at 18 degrees and just sacrifice some streetability. Also, you could look for a "lean burn" slant distributor. Those don't have weights or springs and are essentially "locked" from the factory. How built is your Slant? Is it a forged assembly stock?
Best of luck with the project!